Ok choosing one outdoors is pretty easy. If it is sunny outside, choose 100 ISA or 200 if that is the lowest the camera offers. You can also put it on a sunny mode if your camera has it.

If it is very cloudy, you could stick with 200.

Now if your inside, and you only have the camera flash, do you put it on 400, and 200 if you have an external flash if your subject is within 10 feet? Will the camera usually compensate on auto setting if it knows you have a external flash? With a higher ISA it means less light needs to hit the lens in order to use a smaller aperture setting. Don't know why it's backwards, smaller aperture should mean the opening is smaller, but it's the other way around which confuses us vacation type of photographer.
When would you use a higher setting over ISA 400? I would think higher ISA would allow a faster shutter speed?

[QUOTE=Blueberry;991578]With a higher ISA it means less light needs to hit the lens in order to use a smaller aperture setting. Don't know why it's backwards, smaller aperture should mean the opening is smaller, but it's the other way around which confuses us vacation type of photographer.
QUOTE]

The reason that the aperture gets smaller with a bigger f number is that it is in 'stops', these relate to how much light is stopped from getting into the camera so the higher the f number the more light is stopped so the aperture is smaller to do this.

Adjusting ISO is going to affect exposure so not always meaning fast shutter speed but potentially correct exposure. As with the photo I posted in the above thread, if I had used ISO 400 for example rather than the 1600 I did use then the background would have been a lot darker (going from ISO 1600 down to ISO 400 is 2 stops so 1/4 of the light). This wouldn't have given the feeling to the shot I desired.

Thanks again for your help. I have enough knowledge on photography to be confused. Taking photography class in high school when I shoting black and white photos and developing them in the dark room was a lot of fun and educational back in the 80's. As I am getting more into photography these days (since I have kids) I am trying to take better photos.

First it is ISO not ISA. ISO = International Standards Organization although it is actually the International Organization for Standardization.

“Ok choosing one outdoors is pretty easy. If it is sunny outside, choose 100 ISA or 200 if that is the lowest the camera offers. You can also put it on a sunny mode if your camera has it.”

Okay so far.

“If it is very cloudy, you could stick with 200.”
If it is VERY cloudy, 400 ISO might be more useful.

“Now if your inside, and you only have the camera flash, do you put it on 400”
Or even 800.

“and 200 if you have an external flash if your subject is within 10 feet?”
NO. I use 400 and often 800 ISO even with a powerful external flash. It allows me to include some ambient (available) room light so my flash pics don’t look like I’m shooting in a dark cave. I often combine this with “dragging” the shutter to 1/30. A higher ISO also allows me to have the range and power to bounce flash.

“Will the camera usually compensate on auto setting if it knows you have a external flash?”
Yes, but that doesn’t mean it knows the effect or “look” you want. Depending on the camera it usually sets the SS to 1/60 indoor, higher outdoor (up to 1/500) and the f-stop can vary a great deal. In fully auto mode most cameras will also adjust the ISO.

“With a higher ISA it means less light needs to hit the lens in order to use a smaller aperture setting.”

First let us understand the amount of light that “hits the lens” is not important. It is how much light hits the sensor or film that is important. The distinction is not a linguistic one; there is an important difference in the terminology. i.e. – The amount of light that hits a 70-200mm f2.8 lens may be the same that hits a 70-300mm f4-5.6 lens but the amount of light that gets to the sensor can vary by a factor of four!

“Don't know why it's backwards, smaller aperture should mean the opening is smaller, but it's the other way around which confuses us vacation type of photographer.”

You are confusing a smaller aperture with a smaller f-stop.
“smaller aperture should mean the opening is smaller” – and it is!
“but it's the other way around” – no it’s not.

Let me try to explain. The aperture is an opening inside the lens that looks exactly like the iris in your eye. If you go outside on a Bright Sunny day, a BS kind of day, your iris will get smaller or “close down”. If your camera is in an automatic mode the same thing will occur, the aperture will close down. If you move indoors both your iris and the cameras aperture will open up or have a larger opening to compensate for the lack of light.

What is probably confusing you is the f-stop. The f-stop is a number that represents the aperture opening. This is a list of f-stops commonly found on older film camera lenses.
f 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22.
The higher the number the smaller the opening. Why?, because the number is actually a fraction, therefore f16 would be 1/16, a smaller opening than f4, or 1/4.

This is the same as the shutter speeds where 60 is 1/60th of a second and 500 is 1/500th of a second.

The aperture controls two things: exposure and depth of field.
EXPOSURE: An increase of one aperture will half the amount of light that hits the sensor. A decrease of one aperture will double the amount of light that hits the sensor.
DEPTH OF FIELD: How much of the picture in front of and behind the subject is sharp. The higher the f-stop the more depth of field your will get. The smaller the number the less depth of field.

“When would you use a higher setting over ISA 400? I would think higher ISA would allow a faster shutter speed?”

I use a higher ISO when:
1. There is less light
2. I need a higher SS or
3. I need a higher f-stop.
Every time I double the ISO I gain one f-stop or one SS.

Example – Shooting a basketball game in a local gym gives me the following numbers:
At ISO 100 I get a reading of 1/30 at f2.8. But that SS will cause a lot of my pics to be blurry because I might shake and/or my subjects move while cheering.
So I raise the ISO to 200 and I gain one SS or one f-stop = 1/60 at f2.8. Still having motion problems so I go to 400 ISO and get 1/125 at f2.8.

Fine except I want to take some action pictures of the game and I know I need a SS of 1/500 to “stop action.”

The story behind the ASA, ISO and DIN is interesting (to history or trivia buffs).

If you have a roll of film you will find the ISO is two numbers, not just one. Why?

Many years ago when the Japanese camera manufacturers started to dominate the market they got tired of making two different scales on cameras, one for the U.S. market (ASA) and another for Europe (DIN or Deutschland Industries Norm).

The ASA was arithmetic – every doubling of the number indicated a doubling of the film sensitivity. So ASA 200 was twice as sensitive to light as 100 ASA and half as sensitive as 400 ASA. This corresponded nicely to the change of one f-stop or one shutter speed.

The DIN numbering system was based on a logarithmic scale I still don’t fully understand, but to indicate a double in film sensitivity you added “3”…
24 DIN was twice as sensitive as 21 DIN and half as sensitive as 27 DIN.

So the Japanese went to the ISO folks in France, where they believe everything can be reduced to a number, and tried to get the two systems to agree on a single number.

Z Z Z . . . Nine years later . . . Z Z Z

The ISO for film is released! Here are some examples (the one you will find on an older roll or box of film):
ISO 100/21*, ISO 200,24* ISO 400/27* (the asterisk is a degree symbol)

Look familiar? Nobody would agree to single number so the ISO people gave up and the official number is just a listing of the two numbers with the ASA listed first followed by the DIN.

The Japanese manufacturers were pissed! They gave up and just started using the ASA because they sold more cameras in the US. By default the ISO now uses only the ASA number, but in technical terms is still a two-numbered system.

It just goes to prove there are three kinds of people in the world – those who understand math and those who don’t.

What is probably confusing you is the f-stop. The f-stop is a number that represents the aperture opening. This is a list of f-stops commonly found on older film camera lenses.
f 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22.
The higher the number the smaller the opening. Why?, because the number is actually a fraction, therefore f16 would be 1/16, a smaller opening than f4, or 1/4.

MORE TO COME…

I have found that it is less confusing to newcomers if one uses the correct way of expressing aperture, which would be f/1.4, f/2, ..... f/22. Explain that it is written that way because the number is the result of the focal length divided by the actual aperture. I.E. - 50mm focal length lens with an actual (effective) 25mm diameter aperture is f/2.

I have hesitated about posting on this thread - since I do not want to muddy the waters. Africa has provided an excellent (and perfectly accurate) description of film speed, ISO, ASA and DIN, or however you choose to reference it - and especially how you can apply it in a wide variety of circumstances. The intent is not to confuse - but to introduce a slightly different way to think about the topic, that might be helpful (or not).

In the days of film, you purchased your roll and loaded it - you were somewhat stuck with the indicated film speed of the roll loaded for 12, 24, 36 images, or until you loaded the next roll of film. Now that roll may have lasted a few minutes, days, week or year - depending on how quickly you took pictures.

Fast forward to today and the digital world. Rather than a roll of film, you have an electronic sensor, gathering the light - which is composed of individual pixels arranged on this electronic substrate (the sensor). You now have essentially a setting (ISO speed, or what ever the camera manufacturer labels it) - a parameter to adjust this sensor on a shot to shot basis - that provides much more control.

If we look at the result of varying this one parameter (lets call it ISO for the sake of argument), we essentially see that - to the eye, the setting affects image quality - or expressed another way, the sensitivity of the sensor to light. Now right now - you may be scratching your head asking yourself why would those two be connected? Well, lets look at some examples (and I apologize to using a link to another site)...

If you scroll down to the bottom two images, you see that the ISO 1600 image has more "speckling" or noise in it, as opposed to the one immediately above it (ISO 100), hence the slower ISO image has the appearance of better image quality (none of the speckling on close inspection). So the higher the ISO number the more sensitivity to light is created, which as a byproduct - degrades the image quality. The cause of this speckling, is making the sensor more sensitive to light, thus each of the individual pixels of the sensor that gathers the light, becomes so sensitive, that when light hits the sensor next to it, it too registers some of the light - hence the term "noise", as some of the light signal leaks from one pixel to its adjacent neighbors.

Photographers use this sensitivity just as Africa indicated - on cloudy days, you need more light (or sensitivity) - go to ISO 200, sunny days ISO 100 and so forth. Usually the noise does not start showing up until you are over ISO 400.

So as an example, I like to shoot wide angle landscapes - usually in the evening, with ambient light. So, I choose my best and sharpest wide angle lens - my 12-24, use f/8 for a good depth of field, set ISO 100 for the least amount of noise and best image quality. What is left, well shutter speed - I am up to 15, 30 or sometimes 45 seconds. What am I shooting - lets say some boats moored in a harbor. They tend to bob up and down, so how do I want to compromise to get the intended shot.

I could go to f/4 or something less than f/8 and reduce my depth of field (and adjust the focus for the change in DOF) - reducing shutter speed from 30 seconds to 15 (f/5.6) to 7.5 seconds (at f/4).

I could go to ISO 200 say dropping the exposure from 30 to 15 seconds, or ISO 400, thus going to say 7 seconds. Take a test shot and decide.

I could go to ISO 800 say at 3 seconds and if the noise is too noticeable then in post processing use a noise reduction utility to try to clean it up.

Or vary everything a bit, going from f/8 to f/5.6, and ISO 100 to say ISO 400, in order to speed up the shutter say from 30 seconds to now something on the order of 3 seconds (trying to compromise as little as possible).

So various photographers are going to use and think about ISO speed in ways, that complement what and how they shoot pictures. Yes, I did choose the examples to show another extreme.